On the path to realization of an optical quantum computer — NTT, The University of Tokyo and RIKEN leverage the high speed and broadband nature of optical communications to measure the world's fastest 43 GHz real‐time quantum signal

NTT, the University of Tokyo and RIKEN announced at a press conference that they have jointly developed a new technology to apply the most advanced commercial optical communication technology represented by the fifth‐generation mobile communication systems (5G) to the optical quantum field, paving the way to realizing an optical quantum computer that fully exploits the high‐speed and broadband nature of optical communication technology. This has enabled the world's fastest 43 GHz real‐time quantum signal measured using an optical communications detector. The optical quantum computer uses the traveling waves of photons as quantum bits, and the results of this joint development will bring about a paradigm shift that will revolutionize conventional quantum computer development methods. Moving forward, the group aims to realize a large‐scale super quantum computer with ultra‐high speeds in the 100 GHz band frequency and 100 multi‐core parallelism. Their research was published in Applied Physics Letters.

Various methods have been proposed to create quantum computers. Among these, measurement‐induced optical quantum computers based on time domain multiplexing techniques, which can achieve larger scales and higher speeds, are drawing attention.

This method uses traveling wave qubits (flying qubits), in which photons fly in and out at high speed, rather than standing wave qubits as in superconducting quantum computers. The number of qubits can be expanded by using a portion of the light waves continuously emitted from a quantum light source as qubits and arranging these on a time axis, allowing for larger scale without increasing the size of the device or the integration of elements.

Furthermore, they pair well with optical communication technology and can leverage the highly reliable and high‐performance technology of optical communication that has been developed to date. Moreover, quantum computation at high clock frequencies can be expected by making use of high‐speed optical communication technologies for 5G and other applications.

However, not all high‐speed optical communication devices that have been developed within the realm of classical mechanics can be used directly for optical quantum computers. According to Asuka Inoue, a researcher at NTT Device Technology Laboratories, who presented the results at the press conference, "For example, it is not possible to use high‐speed detectors above 100 GHz for optical communications to measure optical quantum states." She explained that this is because high‐speed optical communication detectors have high optical loss, and that this loss causes the optical quantum state to collapse.

Conventionally, measurements have had to be made with specially designed low‐speed detectors with low optical loss, which limits the clock frequency in measurement‐induced quantum manipulation.

In their research, the group developed an optical parametric amplifier to amplify light while retaining optical quantum information, and also developed a new method to apply the previously inapplicable ultrafast optical communication technology to the optical quantum field. This new technology paves the way for the materialization of a super quantum computer that fully exploits the high‐speed and broadband nature of optical communication technology.

In the experiment, the researchers used a direct‐junction periodically poled lithium niobate (PPLN) waveguide, which has a high amplification factor (approximately 3,000 times) and a small signal‐to‐noise factor (approximately 20%), which NTT has been researching and developing for many years.

Using a 43 GHz detector for optical communications and a real‐time oscilloscope, they measured the amplitude of squeezed light (light with quantum noise compressed) and found that the quantum noise compression rate was about 65%.

This result exceeds the minimum quantum noise compression required for the operation of optical quantum computing (60%), meaning that faster quantum operations can be achieved that can operate at 1,000 times higher clock frequencies than with conventional techniques.

Professor Akira Furusawa, of the School of Engineering at the University of Tokyo (Deputy Director of the RIKEN Center for Quantum Computing (RQC)) and PM of Moonshot Goal 6, explained, "This method requires highly efficient and fast amplitude measurements. This time, the optical signals are amplified as optical signals using optical parametric amplifiers, and we were able to achieve high efficiency without sacrificing high speed, which was sacrificed to achieve high efficiency in conventional methods." The research group is keen to make practical use of this technology, saying that it has established the basic technology for realizing a 43 GHz clock optical quantum computer, and that by combining it with wavelength multiplexing in optical communications, a 100 GHz clock/100‐core super‐quantum computer is attainable.